Add tenant project group definition example and definition in the doc

This patch aims to cover an undocumented tenant configuration
that is the projects group. In configloader.py:TenantParser._getProjects
a project group definition is allowed. A config-projects or
untrusted-projects list could be composed of dictionnaries
with a projects attribute and at least an include or exclude
attribute.

Change-Id: Ia3ca58b4186ee21fed1edd42ecf09ac4355008cc
1 file changed